In the area of switching power supply, constant on time control is widely utilized in the industry due to its excellent performance of transient response, simple structure of control circuit and smooth switching of control method.
According to prior art, a power supply controller applying constant on time control method usually compares an error signal with a triangle signal that is generated according to an input voltage and an output voltage, and the output of the controller triggers a timer to generate a system control signal. During this process, the amplitude of error signal should be matched with the amplitude of the triangle signal for currently triggering the timer. To match amplitude, the triangle signal comprises a fixed direct current (DC) bias in convention. However, a fixed DC bias may constrain the range of the output voltage of the power supply system, and also make it difficult to integrate a soft start function into the controller.
Meanwhile, in prior, the error signal is usually required to change smoothly to make the power supply system stable. When the system is lightly loaded, the system may be operating in a discontinuous-current mode. Under such mode, all power switches in the system may be turned off for a period in each operational cycle. The error signal is gradually declining during this period, which results a relatively larger difference between the error signal and the triangle signal. At this time, the system transient response performance is seriously deteriorated.